1. Field of the Invention
This invention relates to a heating-fixing roller of a fixing device applied to a recording apparatus such as an electrophotographic copying apparatus, a printer or a facsimile apparatus. This invention also relates to a heating-fixing roller as heating means utilized for the fixation of unfixed images.
2. Description of the Prior Art
The heating-fixing roller heretofore put into practical use, has a temperature detector on the surface thereof and is temperature-controlled by changing over a halogen heater provided at the center of the interior of the roller to its electrically energized condition or its electrically non-energized condition.
This temperature control is considerably complicated and therefore, heating-fixing rollers utilizing ceramics of a positive temperature coefficient of resistivity (hereinafter referred to as PTC) characteristic having a self temperature control function have been proposed.
U.S. Pat. No. 4,266,115 discloses a heating-fixing roller using bar-like semiconductor ceramics having the PTC characteristic and formed to have bar like shape. However, the bar-like shaped PTC ceramics are thick and therefore must be provided on the inner surface of the roller in greatly spaced apart relationship with one another. Therefore, in the roller disclosed in U.S. Pat. No. 4,266,115, the surface layer of the roller heated by the ceramics assumes a non-uniform temperature distribution and causes unsatisfactory fixation.
Also, the PTC ceramics disclosed in said U.S. Patent are baked bar-like shaped ceramics and therefore, there occurs a problem when an electrode for imparting an applied voltage to the PTC ceramics is provided. When a current flows from the opposite ends of the bar to generate heat, even if only the central portion of the roller with respect to the axial direction thereof falls in temperature by reason of the heat therein being taken up by paper and toner, the supply of power to that portion is not effected, thereby causing unsatisfactory fixation. This is because the other portion of the roller is at a predetermined temperature or higher and therefore the end portions of the PTC ceramics opposed to each other do not pass the current therethrough due to their self temperature control. Conversely, if an attempt is made to flow a current by applying a voltage from the front surface and back surface of the bar-like shaped ceramics, when fixation is effected continuously, unsatisfactory fixation will be caused because the heat generation of the PTC ceramics is small.
Thus, an electrode is desired which can uniformly maintain the surface temperature of the heating-fixing roller in a practical range. A heating-fixing roller is also desired which will not cause unsatisfactory fixation even for recording materials of different sizes and can accomplish stable fixation.
On the other hand, if an electrode is provided on the surface of ceramics having a PTC characteristic, the ceramics will locally increase in temperature even to 500.degree. C.-1000.degree. C. during the setting of the electrode and therefore, cracks will occur in the ceramics. Heretofore, the ceramics used in the field of fixation have consisted of a binder mixed with powder but are so tattery that they cannot maintain their shape, and it has been unavoidably necessary to bake the ceramics to obtain their shape. Accordingly, the inventor has thought that it is necessary that the conventional ceramics, before being baked, can be readily changed in shape. The conventional ceramics, before being baked, have contained a binder of several % or less relative to the raw powder material and have been undeformable.
Also, the conventional PTC ceramics have sometimes been poor in heat conversion efficiency relative to the power supply and no clear explanation of this problem has been made.